Addition reaction products, obtained by an addition reaction of an unsaturated monobasic acid such as (meth) acrylic acid with an epoxy group present inside a molecule of a polyepoxide such as bisphenol A-based epoxy resin in the presence of a catalyst, and resins obtained by mixing a vinyl monomer therewith, are generally referred to as vinyl ester resins, and all of these are curable resin compositions. These are also referred to as epoxy (meth)acrylate resins.
There have been numerous attempts to modify vinyl esters by adding various polymers to vinyl ester resins conceptually including these epoxy (meth)acrylate resins, and in particular some attempts to improve toughness by adding a powder of minute polymer particles having a core-shell structure are disclosed in Patent Document 1 and Patent Document 2.
In Patent Document 1, for example, a resin composition solution is prepared by mixing and stirring, while heating at 60° C., a resin composition obtained by dispersing minute polymer particles having a core-shell structure, “Kureha Paraloid” EXL-2655, composed of a core portion comprising a copolymer of styrene and butadiene and a shell portion comprising a methacrylic acid-alkyl copolymer, in a liquid mixed resin composition of styrene and a vinyl ester derived from bisphenol A diglycidyl ether with a homomixer manufactured by Tokushu Kika Kogyo Co., Ltd.
In addition, in Patent Document 2, for example, a BMC for synthetic marble is prepared by mixing and stirring 100 parts of a vinyl ester resin, 15 parts of a rubber-containing minute polymer particle having a core-shell structure (average particle size: 0.3 μm, rubber glass transition temperature: −50° C., Zeon Kasei Co., Ltd., product name: F-351), 2 parts of a curing agent (NOF Corp., product name: Percure HOT), 200 parts of aluminum hydroxide (Sumitomo Chemical Co., Ltd., product name: CW-308B), 1 part of magnesium oxide (Kyowa Chemical Industry Co., Ltd., product name: Kyowa Mag #150), 5 parts of zinc stearate (Adeka Fine Chemical Co., Ltd., product name: ZNS-P) and 5 parts of crosslinked polystyrene (Soken Chemical and Engineering Co., Ltd., product name: SGP-70C) with a planetary mixer, followed by aging for 24 hours at room temperature.
In other words, in these prior art, vinyl ester resins containing minute polymer particles having a core-shell structure are obtained by mixing a common commercially available powder of minute polymer particles having a core-shell structure into a vinyl ester.
A powder of minute polymer particles having a core-shell structure is produced by carrying out coagulation/drying treatment on core-shell-type polymer particles in the form of an aqueous latex prepared by emulsion polymerization. Although each of the core-shell polymer particles has been produced typically having a primary particle size of 0.1 μm to 1 μm, when converted to a powder by going through coagulation/drying treatment, a plurality of particles agglomerate to form a powder in the form of agglomerates having a secondary particle size of about 100 μm to 200 μm. Since particles having a size of 0.1 μm to 1 μm have an extremely high physical cohesive force, it is difficult to form a powder in which the particles are separated from one another.
Moreover, it has been necessary to set the glass transition temperature of the shell polymer of minute polymer particles having a core-shell structure to a temperature significantly higher than room temperature. The glass transition temperature of typical shell polymers has been set to 70° C. or higher. Unless this is done, it is well known that the above-mentioned secondary particle size increases considerably, not only causing the powder to become coarse and extend over a wide size range due to, for example, the simultaneous presence of particles of a few centimeters to a few millimeters, but also impairing handleability (e.g. powder blocking). Moreover, as the secondary particle size increases, considerable labor and energy are required to disperse particles to approach a dispersion of primary particles.
Thus, even if a powder of minute polymer particles having a core-shell structure is mixed into a liquid such as a vinyl ester resin and subjected to powerful mechanical shearing force with a homogenizer or the like, it is extremely difficult for the minute polymer particles having a core-shell structure to achieve a uniform dispersion in which particles are separated from one another without agglomeration in the vinyl ester.
In addition, vinyl esters generally contain a large amount of styrene monomers, and from the viewpoints of, for example, odor, safety and preventing the quality deterioration due to unwanted polymerization, mixing with a powder of minute polymer particles having a core-shell structure cannot be industrially convenient. Thus, the technology of modifying vinyl esters with minute polymer particles having a core-shell structure has actually been used in a small number of extremely particular applications.
Patent Document 1: JP-A 2001-098175
Patent Document 2: JP-A 2003-327845